I need to convert Latitude/Longitude coordinates into UTM, in C++, specifying which zone to use.

I am working with locations that fall in two zones, and would like to project them all into one zone - since they are not too far from the border, I am hoping the error introduced by doing so won't be too much.

So far I had been using Chuck Gantz's library, which converts LL to UTM, but it does not support conversion from LL to UTM specifying which zone to use.